docs: def read_vocab(args, column_name):
    """Reads a vocab file if it exists.
  
    Args:
      args: command line flags
      column_name: name of column to that has a vocab file.
  
    Returns:
      List of vocab words or [] if the vocab file is not found.
    """
    vocab_path = os.path.join(args.analysis,
                              feature_transforms.VOCAB_ANALYSIS_FILE % column_name)
  
    if not file_io.file_exists(vocab_path):
      return []
  
    vocab, _ = feature_transforms.read_vocab_file(vocab_path)
    return vocab